Essay Concerning Human Understanding

by John Locke
Book II
Chapter XXVII
Paragraph 9-11

9. Personal Identity. This being premised to find wherein personal identity consists, we must consider what person stands for; which, I think, is a thinking intelligent being, that has reason and reflection, and can consider itself as itself, the same thinking thing in different times and places; which it does only by that consciousness, which is inseparable from thinking, and as it seems to me essential to it; it being impossible for anyone to perceive, without perceiving, that he does perceive. When we see, hear, smell, taste, feel, meditate, or will anything, we know that we do so. Thus it is always as to our present sensations and perceptions; and by this everyone is to himself, that which he calls self: it not being considered in this case, whether the same self be continued in the same, or divers substances. For since consciousness always accompanies thinking, and 'tis that, that makes everyone to be, what he calls self; and thereby distinguishes himself from all other thinking things, in this alone consists personal identity, i.e. the sameness of a rational being: and as far as this consciousness can be extended backwards to any past action or thought, so far reaches the identity of that person; it is the same self now it was then; and 'tis by the same self with this present one that now reflects on it, that that action was done.

10. Consciousness makes personal identity. But it is farther inquired whether it be the same identical substance. This few would think they had reason to doubt of, if these perceptions, with their consciousness, always remained present in the mind, whereby the same thinking thing would be always consciously present, and, as would be thought, evidently the same to itself. But that which seems to make the difficulty is this, that the consciousness, being interrupted always by forgetfulness, there being no moment of our lives wherein we have the whole train of all our past actions before our eyes in one view: but even the best memories losing sight of one part whilst they are viewing another; and we sometimes, and that the greatest part of our lives, not reflecting on our past selves, being intent on our present thoughts, and in sound sleep, having no thoughts at all, or at least none with that consciousness, which remarks our waking thoughts. I say, in all these cases, our consciousness being interrupted, and we lose sight of our past selves, doubts are raised whether we are the same thinking thing; ie the same substance or no. Which, however reasonable, or unreasonable concerns not personal identity at all. The question being what makes the same person, and not whether it be the same identical substance, which always thinks in the same person, which in this case matters not at all. Different substances, by the same consciousness (where they do partake in it) being united into one person; as welll by different bodies, by the same life are united into into one animal, whose identity is preserved, in that change of substances, by the unity of one continued life. For it being the same consciousness that makes a man be himself to himself, personal identity depends on that only, whether it be annexed only to one individual substance, or can be continued in a succession of several substances. For as far as any intelligent being can repeat the idea of a past action with the same consciousness it had of it at first, and with the same consciousness it has of any present action; so far it is the same personal self. For it is by the consciousness it has of it's present thoughts and actions, that is, is self to itself now, and so will be the same self as far as the same consciousness can extend to actions past or to come; and would be by distance of time, or change of substance, no more two persons than a man be two men, by wearing other clothes today than he did yesterday, with a long or a short sleep in between; the same consciousness uniting those distant actions into the same person, whatever substances contributed to their production.

11. Personal identity in change of substances. That this is so, we have some kind of evidence in our very bodies, all whose particles, whilst vitally united to this same thinking conscious self, so that we feel when they are touched, and are affected by, and conscious of good or harm that happens to them, are a part of our selves; i.e. of our thinking conscious self. Thus the limbs of his body is to everyone a part of himself: he sympathises and is concerned for them. Cut off an hand, and thereby separate it from that consciousness, we had of its heat, cold, and other affections; and it is then no longer a part of that which is himself, any more than the remotest part of matter. Thus we see the substance, whereof personal self consisted of at one time, may be varied at another, without the change of personal identity; there being no question about the same person, though the limbs, which but now were a part of it, be cut off.